Work is ongoing in 3GPP, 3rd Generation Partnership Project, and with some operators to specify and investigate the possibility to provide home and/or small area coverage for a limited number of users using a small base station, commonly called a Femto NodeB for WCDMA, Wideband Code Division Multiple Access, or Femto eNodeB (E-UTRAN (Evolved-UMTS Terrestrial Radio Access Network) NodeB) for LTE, Long Term Evolution. Other commonly used names are HNB (Home NodeB) for WCDMA and HeNB (Home eNodeB) for LTE. The femto solutions based on WCDMA technology are called WCDMA Femto solutions and the solutions based on LTE technology are called LTE Femto solutions. Similar solutions can also be deployed for other radio technologies, like GSM. In this application, the name Femto NodeB is used as an exemplary name for the small base station and should not be read to limit the scope only to WCDMA Femto solutions. In the same way, the term Macro NodeB is used as an exemplary name for any traditional base station using any technology. The Femto NodeB would provide normal LTE/WCDMA/GSM coverage for the end users, a so called femto cell, and would be connected to the mobile operator's network using some kind of IP based transmission. One example is to use fixed broadband access (e.g. xDSL or Cable) to connect the Femto node to the mobile operator's network. Another alternative would be to use Mobile Broadband access, for example, High Speed Downlink Packet Access, HSDPA, and Enhanced Uplink.
There are several ways to use the available radio frequencies between the femto layer and the traditional cellular deployment layer, referred to as ‘macro’ layer even though it may comprise both macro, micro and pico cells. There are three main Channel Deployment Scenarios (CDS) that are relevant, namely:                CDS1: One frequency is used by both the Femto NodeBs and the Macro NodeBs. This CDS has some severe interference problems that will make it hard to work.        CDS2: One dedicated frequency is used only by the Femto NodeBs and (at least) another frequency is used only by the Macro NodeBs. This CDS is the preferred one if only technical arguments are taken into account. However, it is very unlikely that operators are willing to dedicate whole frequencies for the femto layer, mostly due to economical reasons.        CDS3: One frequency used both by the Femto NodeBs and the Macro NodeBs and (at least) another frequency used only by the Macro NodeBs.        
CDS3 is expected to be a common way for deployments. This deployment scenario may as well as CDS1 cause different types of interference in certain scenarios, both between the femto and the macro layer and internally in the femto layer as it is assumed that all the Femto NodeBs are using the same frequency.
By using frequency deployment scenario CDS3, as well as by using CDS1, it is possible that different types of interference is created when a user equipment (UE) camped on the macro network in the first cell on the first frequency approaches a second cell, a femto cell on a second frequency and the first and second frequencies are the same. The following scenario describes the different types of interference that may arise in this case and applies mostly for WCDMA Femto, but may also apply for LTE Femto as well as for GSM Femto solutions.
An UE active or camped on a femto cell is called Femto UE in this application. In a similar way, an UE active or camped on a macro cell is called Macro UE. In a scenario, the Macro UE is camped on the macro network on a first macro cell on the same frequency as the second femto cell, that is, the first and second frequencies are the same. The Macro UE enters the area of femto cell coverage and if the Macro UE is not allowed to use the femto cell for any reason, for example, based on Access Control as part of idle mode behavior, it may continue to camp on the first macro cell that is on the same frequency as the second femto cell. If the Macro UE later enters active or connected mode, it will create interference towards the femto cell and the Femto UEs using that cell. Depending on the location of the Macro UE, this may lead to very poor performance of the interfered Femto NodeB.
In a similar way, the femto cell, that is, the Femto NodeB and the Femto UEs on that Femto NodeB, may create interference towards the Macro UE. For example, if a Femto UE is currently in idle mode and would later enter active mode, then interference would be created towards the Macro UE.